Programmable defibrillator

ABSTRACT

The subject invention integrates a two-channel defibrillator with a programmable stimulator to provide a means for assessing lethal ventricular tachyarrhythmias and determining defibrillation thresholds during implantable defibrillator procedures. The subject apparatus includes a number of features to aid doctors as well as improve patient care at substantially decreased patient risk. These features include an automatic charging circuit, as well as dual channel high voltage capacitor circuits to reduce the time in which a rescue shock can be delivered to a patient after an initial test defibrillation shock. Parameter storage is provided to allow the unit to be preprogrammed prior to the initiation of an electrophysiologic procedure. A microprocessor controlled display system provides the physician with information parameters regarding defibrillation shocks. This displayed information includes the energy delivered and the resistance in the patient. In addition, the apparatus also displays information regarding energy which is expected to be delivered based on the entered parameters.

This is a continuation of co-pending application Ser. No. 07/301,729filed on Jan. 26, 1989, which is a divisional of U.S. Ser. No.06/865,181 filed May 14, 1986 and now U.S. Pat. No. 4,827,936 issued May9, 1989.

TECHNICAL FIELD

This invention relates to an apparatus for use in the field of cardiacelectrophysiology. More specifically, an apparatus is disclosed forassessing ventricular tachyarrhythmias and determining defibrillationthresholds during implantable defibrillator procedures.

BACKGROUND OF THE INVENTION

In the United States, heart disease is a major health problem. Of the1.5 million people per year who suffer a myocardial infarction, about680,000 survive that have ischemia (dead heart tissue) which is thebasis for cardiac arrhythmias. Approximately 400,000 people a year diefrom the most serious types of cardiac arrhythmias.

Arrhythmias can be classified into three broad types. Bradycardia is anabnormally slow heart rhythm. This problem has been successfully treatedfor a number of years with implantable pacemakers which induce the heartto beat at a faster, normal rhythm. The remaining types of arrhythmiasare more difficult to control.

Tachycardia is a rapid cardiac rhythm generally defined as a heart rategreater than 100 beats per minute. There are normal physiologictachycardias due to exertion or emotion as well as abnormalnonphysiologic tachycardias in which a high rate results in loss ofblood pressure. Sustained ventricular tachycardia can result in severeloss of blood pressure, loss of consciousness and can deteriorate intoventricular fibrillation which is fatal if not quickly interrupted.

Fibrillation, unlike tachycardia, is a disorganized cardiac rhythmwherein the heart quivers rather than beats. This quivering is a resultof multiple waves of cardiac depolarization spreading and collidingthroughout the ventricular tissue. Ventricular fibrillation results in aprecipitous decrease in blood pressure followed quickly by brain damageand death.

Arrhythmias are treated using either medication, surgery or implantationof a medical device. Drug therapy is employed initially in the majorityof cases and involves the use of various medications to prevent anarrhythmia from starting or being sustained. The main advantage of drugtherapy is that no surgical intervention is required. The major drawbackto the exclusive use of therapy is the lack of backup therapy toterminate a potentially lethal arrhythmia should the drug eventuallyfail to prevent the arrhythmia from recurring. Additionally, inattempting to achieve adequate tachycardia prevention, drug related sideeffects often preclude using an adequate dose of medication.

Surgery involves locating the cause of the arrhythmia and removing orisolating it from the healthy cardiac tissue. The advantage of surgicaltherapy is that the procedure is curative when successful. Thedisadvantage of surgical therapy is the morbidity and mortalityassociated with open heart surgery and the technical difficulty and highcost of the procedure. These factors have restricted the practice ofantiarrhythmia surgery.

In 1980, the first implantable defibrillator was implanted in a humanpatient. Implantable defibrillators sense fibrillation and automaticallydeliver a high energy pulse. Subsequent studies have indicated thatthese devices are effective in preventing sudden death fromfibrillation. Presently, no single implantable device has been developedto control all three types of arrhythmias.

The analysis of patients who have arrhythmias often requires invasivetesting in an electrophysiology lab. This invasive testing is carriedout in a variety of situations. For example, invasive testing is commonduring a selection process used to determine which patients might becandidates for implantable defibrillators. Invasive testing is alsoutilized in trying to assess and characterize tachycardia which is thentreated with drugs. In any case, in the testing process, catheters areinserted into the heart and the patient's arrhythmia is provoked withprogrammed electrical stimulation. When the arrhythmia manifests itself,the physician attempts to terminate it with antitachycardia pacing.Antitachycardia pacing is described in the literature and consists of aseries of low voltage pulses designed to reset the normal heartbeat. Ifpacing fails, the patient is either cardioverted with a substantiallyhigher voltage shock or defibrillated with a very high voltage energypulse.

Low energy cardioversion utilizes pulses with energy levels far greaterthan pacing pulses but lower than high energy defibrillation pulses.With energies of less than 5 joules, this mode of therapy is based onthe theory of interrupting the arrhythmia by stimulating the tissue,rendering it nonexcitable. Low energy cardioversion has been clinicallydemonstrated as effective. Its drawbacks include patient discomfort andthe fact that improperly timed pulses can accelerate tachycardias andoccasionally induce fibrillation.

In contrast, high energy defibrillation uses pulses with energy levelstens of thousands of times greater than pacemaker pulses. High energydefibrillation is accomplished by stimulating a large portion of theventricular tissues simultaneously and rendering it nonexcitable,thereby terminating the arrhythmia. If a patient is found suitable, aninternal defibrillator can be implanted to control the arrhythmia.During the operation, the patient's defibrillation threshold must bedetermined. First, the patient is fibrillated using a programmablestimulator, then a special defibrillator is used to determine the energyrequired to defibrillate the patient.

The invention described herein facilitates the assessment of arrhythmiasand defibrillation thresholds resulting in improved patient care andsubstantially decreased patient risk. In the prior art, there existedboth programmable stimulators and cardioversion/defibrillator devices.The programmable stimulator includes a means to pace the patient's heartwith critically timed stimuli to provoke the cardiac arrhythmia. Thesedevices are then used to terminate the arrhythmia using antitachycardiapacing. If the pacing accelerates the arrhythmia or fails to terminateit, then a standby defibrillator device must be set up and the patientcardioverted or defibrillated. Frequently, the patient is cardiovertedor defibrillated externally. More recently, internal catheters have beenprovided to deliver the defibrillation shock.

As noted above, an external cardioverter/defibrillator is used duringimplantable defibrillator procedures to assess the patient'scardioversion/defibrillation threshold. When the patient is fibrillatedusing a programmable stimulator, the unit must be disconnected before atest defibrillation pulse can be applied. Frequently, the test pulsefails to defibrillate the patient. Once the physician recognizes thefailure to defibrillate, he must program a new, higher voltage rescueshock into the defibrillator. The unit must then recharge prior todelivery of the rescue shock. This procedure takes considerable time andthere is ample opportunity for operator error. Any delay indefibrillating the patient is a serious health risk and improving theresponse time to the delivery of the rescue shock substantially reducespatient risk. Accordingly, it would be desirable to eliminate anyunnecessary time between delivery of the test shock and rescue shock.

In the above described procedures, it is also clear that in theelectrophysiology lab, it is frequently necessary to use both aprogrammable stimulator and a cardioverter/defibrillator. Present dayequipment requires the operator to switch leads and move back and forthbetween two pieces of equipment. During this procedure, care must betaken to prevent any of the high voltage charge delivered by thedefibrillator from reaching the output leads of the programmablestimulator to avoid damaging the latter. Accordingly, it would bedesirable to provide a single combination test unit wherein leads wouldnot have to be changed and automatic protection of programmablestimulator would be provided.

Another drawback of the defibrillation devices available in the priorart relates to the fact that little or no measurement and visualfeedback is given to the surgeon regarding the defibrillation pulse.More specifically, the surgeon typically sets a pulse width and avoltage level for a test shock. If this test shock fails, the surgeoncannot be sure whether it was the result of shorted leads, anunexpectedly high resistance in the heart or whether the voltage wasjust too low to stop the defibrillation. There presently exists some lowvoltage pacing devices which have been designed to provide additionalinformation to the surgeon regarding patient resistance and energydelivered. However, to date, no systems have been provided to calculateand display this information in a defibrillation setting. In alife-threatening situation, such as cardiac fibrillation, suchinformation is extremely important and can aid the surgeon in assessingthe type of rescue shock necessary to end the fibrillation.

The energy delivered to the heart of a patient is generally measured injoules. The energy level of the shock is analogous to a dosage intherapy. In prior art devices, as in the subject invention, the surgeonsets the defibrillation shock by adjusting a voltage level and the pulsewidth. However, in the prior art devices, no information is given to thephysician as to the energy which will be received by the patient if ashock with those set parameters were delivered. Therefore, it would bedesirable to provide a device which displays the estimated energy basedon the set voltage level and pulse width.

Accordingly, it is an object of the subject invention to provide a newand improved apparatus for electrophysiology testing in patientssuffering from severe ventricular arrhythmias.

It is another object of the subject invention to provide a new andimproved apparatus which advantageously combines a programmablestimulator and an internal cardioversion/defibrillation device.

It is a further object of the subject invention to provide a combinationstimulator/defibrillator apparatus with automatic circuit protection forthe stimulator.

It is a still another object of the subject invention to provide a newand improved defibrillator which includes a pair of capacitor bankspermitting the simultaneous storage of both a test shock and a rescueshock.

It is still a further object of the subject invention to provide a newand improved defibrillator apparatus which includes automatic rechargecircuitry to reduce the time necessary to deliver a rescue shock duringan emergency procedure.

It is still another object of the subject invention to provide a new andimproved apparatus which allows multiple, independent entries of datawhich are stored for later recall during testing procedures.

It is still a further object of the subject invention to provide a newand improved defibrillator apparatus which will display measurement ofresistance and energy delivered during a defibrillation shock.

It is still a another object of the subject invention to provide a newand improved defibrillator apparatus which will display the energy whichis estimated to be delivered if a shock of a given voltage and pulsewidth is to be delivered.

SUMMARY OF THE INVENTION

In accordance with these and many other objects, the subject inventionprovides a single device that can be used in electrophysiology labsduring ventricular tachycardia procedures. The invention integrates aninnovative two-channel defibrillator with a programmable stimulator in amanner that enhances response time to the patient and provides moreinformation to the surgeon. A number of new features have been includedwhich address unique problems encountered when treating severearrhythmia.

In use, an indwelling defibrillating catheter with pacing capabilitiesis inserted into the heart. In this manner, the arrhythmia can beinduced by the subject apparatus and, if necessary, the patient can bedefibrillated within seconds of the initiation of the arrhythmia. Inaccordance with the subject invention an automatic interrupt means isprovided to protect the delicate circuits of the stimulator from thehigh voltage discharge delivered by the defibrillator without shuntingcurrent to the patient.

Another advantage of the unique combination found in the subjectinvention is that the time necessary to initiate defibrillation is muchshorter than is currently possible, resulting in decreased patient risk.To enhance the rapidity of defibrillation, the unit always remainscharged to the programmed defibrillating voltages.

Devices which exist in the prior art all have a charge button that mustbe pressed to initiate the charging of the storage capacitors. Thisfeature was generally provided because the devices were subject to falsetriggering wherein the high voltage would be released inadvertently. Inrecent years, more reliable equipment has been developed which is notsubject to false triggering. Nonetheless, the prior art devices stillincorporate both a charge button (which is depressed to load thecapacitor) and a separate switch, which must be subsequently depressed,to the deliver the shock after the capacitor has been charged. In theemergency situation of a patient fibrillation event, the additional stepof having to press a charge button can be delayed or overlooked. Even ifthe charge button is properly pressed, time will elapse before thecapacitor bank is raised to the level of the desired shock. In thesubject invention, the charge button is eliminated and an automaticcircuit is provided to maintain the capacitor banks at the set voltagelevel.

The defibrillation circuitry of the subject apparatus includes twoindependent channels. Each channel includes its own storage capacitorswhich can be independently programmed. Prior to a surgical procedure,the surgeon can program one channel with a test shock and the otherchannel with a much stronger, rescue shock. The independent capacitorarrays will automatically be charged to these two independent levels.

In the lab, the test shock can be used to attempt defibrillation. Ifthis test shock does not revert the fibrillation, the rescue shock canbe delivered immediately. This is in sharp contrast to any existingdevice where 20 or 30 seconds might elapse before a rescue shock can beapplied.

A memory capability is also provided to instantly change the programsettings of the two defibrillation channels to previously selectedvalues. Since the charging of the high voltage capacitors is automatic,this provides an effective way to quickly change to a maximum energysetting if the initial two shocks fail to defibrillate the patient. Asimilar memory capability is included in the programmable stimulator. Bythis arrangement, when the electrophysiologist induces the patient'sarrhythmia he can instantly go to a previously selected set ofantitachycardia pacing parameters and thereby attempt to terminate thearrhythmia without the delay of further programming.

As noted above with the prior art devices, if a test shock fails todefibrillate the patient, there is no information presented to thephysician to help him alleviate the problem. The subject inventionprovides a means to calculate and display such information. Morespecifically, in conjunction with each defibrillation shock, the unitwill display to the physician the actual energy, measured in joules,delivered to the patient, as well as the patient's electricalresistance. If the resistance is abnormal, this could indicate a problemwith the electrode system. Without the displayed information, aphysician might not be alerted to the problem thereby compromisingpatient's safety. In a preferred embodiment, the residual charge of thecapacitor is measured and is used to calculate the energy delivered andpatient resistance.

As another aid to the physician, the subject invention also provides ameans for calculating and displaying the energy which is estimated to bedelivered during a defibrillation shock. The energy delivered will varybased on a variety of parameters, such as the voltage level, pulse widthand patient resistance. All of these parameters are used by theapparatus to calculate the expected energy to be delivered. By thisarrangement, the physician can best gauge the proper voltage and pulsewidth settings needed to deliver the desired energy level shock.

Further objects and advantages of the subject invention can beappreciated by referring to the following detailed description taken inconjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram of the apparatus of the subjectinvention.

FIG. 2 is a diagram of the layout of the front panel of the apparatus,illustrating the various input systems and displays.

FIG. 3 is a diagram of the rear panel of the apparatus of the subjectinvention.

FIG. 4 is a flow chart illustrating the steps of delivering high voltagedefibrillation shocks.

FIG. 5 is a flow chart illustrating the automatic charging feature ofthe subject invention.

FIG. 6 is a schematic diagram illustrating the high voltage regulatorused to control and measure the energy in the storage capacitors.

FIG. 7 is a flow chart illustrating the ability to display alternateparameters which are stored in memory.

FIG. 8 is a flow chart illustrating the steps used to calculate anddisplay energy and resistance.

FIG. 9 is a flow chart illustrating the steps used to calculate anddisplay of estimated energy.

FIG. 10 is a schematic diagram of the high voltage protection circuit.

FIG. 11 is a schematic diagram of the high voltage output circuit.

FIG. 12 is a schematic diagram of the programmable stimulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a system block diagram of theapparatus 10 of the subject invention. The apparatus 10 ismicroprocessor controlled. In the preferred embodiment, a 68HC11microprocessor, manufactured by Motorola, is used.

A microprocessor board 20 includes standard components, such as aprogram ROM, RAM, bus drivers and latches. The front panel 22, whichwill be discussed in greater detail below, interfaces with themicroprocessor board 20 through a support board 24. Support board 24includes display drivers, switch decoders, an optical encoder, and otherstandard interface circuits. The microprocessor writes information tothe display drivers and receives information through the optical encoderand switch matrix interfaces. The elements in boards 20 and 24 arestandard and any suitable alternatives can be utilized.

The subject apparatus includes six other principal boards whichcommunicate with the microprocessor. A pair of high voltage regulatorboards 30 and 32 are programmed by the microprocessor to charge the highvoltage capacitors 34 and 36 to the voltage entered on the front panel22 by the physician. Details of the high voltage regulators will bediscussed below with reference to FIGS. 5 and 6. After defibrillation,data is read back from the regulator boards that contain information onenergy delivered and the patient's electrical resistance. This functionwill be discussed below with reference to FIG. 8.

Output boards 40 and 42 include high voltage electrical switches toconnect patients to the high voltage energy storage capacitors throughthe rear panel 43 for the duration selected on the front panel 22 by thephysician. The output boards will be discussed in greater detail inconjunction with FIG. 11.

The subject apparatus further includes a programmable stimulator board44. The programmable stimulator board also interfaces with themicroprocessor 20 to generate the pulses that are programmed into thefront panel by the physician. A more detailed description of theprogrammable stimulator board will be made with reference to FIG. 12.

During a defibrillation attempt, the output from the programmablestimulator is protected through a circuit on board 46. If thisprotection were not available, the output of the stimulator 44 could beeasily damaged. A more detailed description of the protection circuitwill be made with reference to FIG. 10.

The apparatus 10 is powered by a 12 volt rechargeable battery 50connected to a power supply regulator board 52. Board 52 regulates thesystem power supply and provides a signal to the microprocessor boardwhen the batteries need recharging.

The operation of the subject apparatus 10 will be described now withreference to the front panel inputs shown in FIG. 2 along with variousaccompanying flow charts where necessary. In the preferred embodiment,the front panel includes an array of LCD displays, push-button switchesand an optical encoder 60. The basic operation calls for the physicianto depress the switch next to the parameter he wants to adjust. Theselected switch lights up indicating that the optical encoder 60,labelled "parameter adjust", will control the associated parameter. Byrotating the knob 60, the parameter next to the lighted switch will bevaried. As can be seen from FIG. 2, the left portion of the front panelcontrols the defibrillator operation, while the right portion controlsthe programmable stimulator.

All electrical hookups are routed to the rear panel 43 as shown in FIG.3. The two channels of the programmable stimulator terminate in standardBNC connectors 62A and 62B (labelled CH1 and CH2). Used separately,pacing pulses can be delivered to different parts of the heart, althoughthe outputs may be ganged together.

The high voltage outputs 64A and 64B, (labelled P1 and P2) are highvoltage connectors which can be left separate or can be shorted togetherusing a toggle switch 66. When the outputs are shorted together, theoutput from both channels is delivered through the P1 output. FIG. 3also shows an input 67 to receive a charge line to recharge the storagebatteries. The charger is controlled by switch 68.

In order to deliver the P1 shock, the physician refers to the LCDdisplay block 69, labelled "OUTPUT" on the front panel. As illustratedin FIG. 2, P1 has been selected and is displayed in block 69. If P2 isdisplayed and the physician wants P1, he can press the output switch totoggle to P1. The "parameter select" switch below the output switch willbe discussed in greater detail below.

After the P1 shock has been selected, the electrophysiologist can verifythat the voltage and pulse width are correct. This information isdisplayed in the top display block 70 of FIG. 2. As discussed above,these parameters can be adjusted by depressing the associated switch androtating the encoder knob 60. If the parameters are changed, anautomatic charging circuit discussed below will adjust the voltage inthe capacitor bank to the proper level. When charging is complete, aready light 72 will be illuminated, indicating that the charge can bedelivered.

In accordance with the subject invention, prior to initializing a testprocedure, a second, rescue shock can also be programmed. In this case,the surgeon would also adjust the parameters for the P2 shock. In theillustrated embodiment, the parameters are shown in display block 74. Ina typical situation, as illustrated herein, the second rescue shock willhave significantly greater voltage and a longer pulse width since itwould be assumed that the first defibrillation shock failed to revertthe fibrillation.

In use, the patient can be defibrillated by pressing the deliver switch76 which will deliver the voltage stored in the P1 capacitor 34. If thisshock fails to defibrillate the patient, the "OUTPUT" button isdepressed causing the associated display to toggle to P2. The deliverbutton is then depressed and the energy in capacitor 36 will beimmediately delivered.

FIG. 4 is a flow diagram that describes the software in themicroprocessor used to deliver the P1 and P2 charges. As shown therein,the front panel switches are continually scanned in steps 100 and 102.If any switch is pressed, priority is given to the deliver switch instep 104. If the deliver switch has not been depressed, the otherswitches will be serviced in step 105. If the deliver switch 76 isdepressed, the outputs of the programmable stimulator will be protectedvia the disconnection step 106.

The circuit 46 for disconnecting and protecting the programmablestimulator is shown in FIG. 10. Any signals from the stimulator mustpass through this circuit before reaching the rear panel. Normally, themicroprocessor leaves this circuit on by setting the ON/OFF line 140 tothe RF oscillator 141 high.

The RF oscillator 141 couples energy to the gates of the high voltageswitches 142 (Motorola MTM5N100 MOSFETs) through a small pulsetransformer 144. To turn the switches 142 off, the microprocessor setsthe ON/OFF 140 line low, disabling the RF oscillator. Thesource-to-source connection of the MOSFETs along with the transformerisolation of the gate drives 146 results in a symmetric (+,-) 1000 voltprotection. In this arrangement, the high voltage from the defibrillatorpulse is not shunted back to the patient through the pacing leads sincethe disabled MOSFETs define an open circuit. This approach is thereforesuperior to a more simple shunt circuit, such as a Zener diode placedacross the pacing leads. In the latter circuit, the stimulator would beprotected, but the high voltage would be shunted to the pacing leadssuch that the characteristics of the defibrillation shock deliveredwould change, which can reduce efficiency and could damage heart tissue.The ground line from the programmable stimulator also goes through a setof protection switches to avoid defibrillation current shunting duringhigh voltage output, as shown in FIG. 12.

After the programmable stimulator is disconnected, the front panelparameters are checked in step 108 and the proper pulse width for theassociated pulse is selected in steps 110A or 110B of FIG. 4. Themicroprocessor then commands the appropriate output board to connect thepatient to the previously charged high voltage capacitor in step 112A or112B.

FIG. 11 is a block diagram of the output board 40 for capacitor P1. Todeliver a defibrillation pulse the microprocessor turns on the RFoscillator 150 for the programmed duration. The series-paralleled IGFETs152 (Motorola MTP20N50) provide a 50 amp drive capability with lowoutput leakage. While the RF oscillator 150 is on, the transformer 154couples a square wave of voltage from its primary to its secondary. Thisvoltage is rectified, and held by the gate capacitance of the IGFETs.When the RF oscillator is turned off, a pull down resistor 156discharges the gate capacitance turning off the IGFETs. When the IGFETsare on, the ground connection is made to the patient through lead 158,and the high voltage capacitor is connected through the other lead 160.The current is led through high voltage diodes 162 in series with thepatient. The diodes provide reverse voltage protection for the IGFETs.

After the charge has been delivered and prior to the automaticrecharging of the capacitor 36, the residual voltage on the capacitor ismeasured in step 130A or 130B. In the preferred embodiment, residualvoltage is measured to permit the calculation of resistance and energydelivered to the patient as discussed in greater detail below. After theresidual voltage has been measured, the capacitor is automaticallyrecharged in step 132 shown in FIGS. 4 and 5.

As can be appreciated, by having two independent programmable capacitorarrays, a test shock and a rescue shock of different voltages can bedelivered with virtually no time delay therebetween. In use, thephysician will observe whether the test shock has succeeded indefibrillating the patient and if that has failed, a second, highervoltage rescue shock will be delivered.

Another unique aspect of the subject invention which reduces the timenecessary to respond to a critical situation concerns the automaticrecharging of the capacitors. As noted above, all prior art devicesrequired that the voltage be set and thereafter a charge button bepressed to raise the capacitor bank to the desired level to avoid falsetriggering problems. In applicant's invention, as soon as the parametersare entered into the device, both the capacitor banks 34, 36 will becharged to their set levels. In addition, as soon as a shock isdelivered, and after the residual voltage has been read, the capacitorswill begin to immediately recharge.

The steps taken by the microprocessor to carry out this automaticcharging are shown in FIG. 5 and the regulator circuit itself is shownin FIG. 6. As shown in FIG. 5, the front panel parameters arecontinuously scanned in steps 200 and 202. If any parameters have beenchanged, the microprocessor will determine if the high voltage (P1 orP2) parameter has changed as shown in step 204. A change in the highvoltage parameter could result from data entered via the optical encoder60. The setting could also be changed as a result of pressing theparameter select switch 80A and will be discussed below. In any event,if the high voltage parameter has not changed, the other switches willbe serviced as shown in step 206. If the high voltage parameter haschanged, the processor will determine if it has increased in step 210.

If the high voltage parameter has increased, the capacitor will becharged, while if the parameter has been decreased, the capacitor willbe discharged. As shown in FIG. 5, if the capacitor has been dischargedby delivering its energy to the patient, the processor will instruct theregulator board to charge the capacitor as indicated by the input 132,also shown in FIG. 4.

If the capacitor is to be charged, charge line 260 on the regulatorboard shown in FIG. 6 will be set high. If the capacitor is to bedischarged, discharge line 262 will be set high. In conjunction withsetting the charge or discharge lines, the microprocessor will also loadthe voltage read from the selected switch (P1 or P2) into the digital toanalog converter (DAC) 264 in step 216. A comparator 266 compares thehigh voltage from the storage capacitor (P1 or P2) to the output of theDAC 266. Note that the output from the capacitor bank is divided downthrough a resistor array 268 prior to entering the comparator.

The output of comparator 266 is fed back to the microprocessor whichdetects when the output of the capacitor matches the output of the DAC266 in step 218. The charging or discharging is then halted. As seen inthe circuit diagram of FIG. 6, where the capacitor is being charged, lowvoltage from battery V_(BATT) is supplied to a low to high voltage DC toDC converter 270 to charge the storage capacitor.

As pointed out above, another unique advantage of the subject inventionis the ability of the microprocessor to store a number of presetparameters. In this manner, the physician can program all the necessaryparameters prior to initiating the surgical procedure. This ability isprovided in both the programmable stimulator and defibrillator sectionsof the apparatus. As illustrated in the front panel in FIG. 2, bothsides of the display include "SELECT" buttons 80A and 80B. By pressingeither button, the display and electronics toggle between parameter setA and parameter set B. Each parameter set is independently adjustableand all the information is retained in the RAM on the microprocessorboard 20.

The steps taken by the microprocessor in relation to this selectionprocess are shown in FIG. 7. More particularly, the front panel switchesare scanned in steps 300 and 302. If a switch other than the selectswitch 80 is pressed, as determined in step 304, the other switches willbe serviced as shown in step 305. If a select switch 80 has beenpressed, the microprocessor determines what is currently being displayedin step 306. This current display is then changed in step 308A or 308Bto the alternate set of parameters.

The microprocessor also functions to reprogram the hardware in step 310.Thus, for example, in the defibrillator section, the voltages in thecapacitors will be reprogrammed to the new P1 and P2 voltage levels. Therecharging will be automatic, enabling the physician to respond quickerto a particular situation.

It should be noted that the combination of the multiple storageparameters as well as the automatic charging feature and two channelcapacitor array interact to improve patient care. More particularly, ifa patient has gone into fibrillation, the physician can deliver a P1pulse from parameter set A as a test shock. As soon as that shock hasbeen delivered, the P1 capacitor begins recharging. If a surgeondetermines that the first shock was insufficient to revert thefibrillation, the P2 shock from parameter set A can be given. If the P2shock is also insufficient to revert the fibrillation, the physician canthen select parameter set B. As noted above, prior to this selection thecapacitors would already be recharging. The device would then have tomerely complete the automatic recharging of the capacitors to the newlevels of parameter set B and then the ready light will illuminate,permitting the physician to deliver a third shock. In the prior artdevices, 20 or 30 seconds elapsed between each successive shock, whereasin the subject invention, this time period can be significantly reduced.

The ability to enter and store two sets of parameters is alsoadvantageous when using the programmable stimulator. As illustrated inblock 84 of FIG. 2, a series of pulses have been programmed as parameterset A. As seen in block 85 all the pulses are programmed to have a 4.8volt amplitude and a 0.45 millisecond pulse width. The parameter set Apulse series includes 8 pulses, each spaced 500 milliseconds apart (S1)followed by another pulse in 450 milliseconds (S2). The channel 2 pulses(which may be sent along the channel 1 line to the same spot in theheart) provide two more pulses each spaced apart at 400 and 350milliseconds, respectively. This sequence is a typical of one intendedto induce a tachycardia episode.

Parameter set B can be programmed to have a series of pulses designed torevert the tachycardia. For example, a group of eight, similar pulses,spaced apart 300 milliseconds can be used to try to revert thetachycardia. The exact pulse train which is used to revert thetachycardia will vary based upon the patient. It should be noted,however, that the subject invention allows the physician greatflexibility in programming the pulses. Furthermore, if the intendedreversion pulses fail, the surgeon can immediately deliver adefibrillation pulse from the same device along the same leads. As notedabove, in this case, the programmable stimulator leads will beautomatically disconnected, preventing damage.

The subject apparatus is also provided with a "PAUSE" switch 82 shown onthe front panel in FIG. 2. When this switch is off, the series of pulsesin block 84 will be delivered once, each time the start button 86 isdepressed. If the pause switch is on, the series will be deliveredrepeatedly, every 10 seconds, giving time for the physician to alter theparameters in block 84 between each delivery. In this way, small changescan be made until a tachycardia is induced. Switch 87, labeled "S1", isused to set the number of S1 pulses which will be delivered.

FIG. 12 is a simplified block diagram of the programmable stimulator. Inoperation, the microprocessor writes the desired pacing voltage to a DAC320 in HEX format. The output of the DAC is buffered with an op-ampfollower circuit 322 to increase its output current drive capability.When the START button 86 is pressed, the microprocessor paces the heartby sending a pulse of the programmed width (as selected on the frontpanel) to the PACE line 324. The pulse width is timed using the timerbuilt into the 68HC11 microprocessor. After timing the first pulse, themicroprocessor times the programmed pulse-to-pulse interval (500 msecfor S1, as shown on the front panel; 450 msec for S2; etc.). After eachpulse-to-pulse interval, the microprocessor stimulates the heart byactivating the PACE line for the programmed pulse width.

As described above, when defibrillating a patient, it is advantageous toprovide the physician with information regarding the patient'sresistance and the energy delivered by the pulse. This information isdisplayed in the front panel shown in FIG. 2 in blocks 88 and 90. Thisinformation could be derived, for example, by measuring the currentdelivered during defibrillation and then calculating the energy andresistance. In the preferred embodiment, energy and resistance arecalculated by measuring the residual voltage on the discharge capacitor,as noted in step 130 of FIG. 4.

Referring to FIG. 8, the steps taken by the microprocessor forcalculating and displaying energy and resistance are illustrated. Morespecifically, after the high voltage is delivered in step 400, theresidual voltage on the capacitor is measured.

The residual voltage on the capacitor is measured by setting to low boththe charge and discharge lines 260, 262 on the regulator board shown inFIG. 6. The residual voltage of the capacitor will be supplied to thepositive lead on the comparator 266. The microprocessor then varies theoutput of the DAC 264 in a successive approximation fashion. Morespecifically, if the voltage output from DAC 264 is higher than theinput from the capacitor, the comparator output will be low and themicroprocessor will drop the voltage output of the DAC. If the newoutput from the DAC is not low enough to change the output from thecomparator, further reductions in the output will be made until thecomparator output toggles to high. Then the voltage from the DAC will beincreased until the comparator output toggles back to low. Thisprocedure Will continue until a change in the least significant bit ofthe code to the DAC is enough to toggle the output of the comparator. Atthis point the code in the DAC represents the residual voltage on thecapacitor voltage.

After the residual voltage on the capacitor is measured, themicroprocessor begins the recharging procedure 404 as discussed indetail with regard to FIGS. 5 and 6. The microprocessor will alsocalculate the energy and resistance delivered in step 406.

The energy delivered in the shock can be calculated using the followingequation:

    J=0.5C(Vi.sup.2 -Vf.sup.2)                                 (1)

where Vi equals the initial voltage, Vf the residual voltage, C thecapacitance of the high voltage capacitor, and J the energy delivered injoules.

Resistance R can be calculated by the following equation:

    R=-PW/[(C)1n(Vf/Vi)]                                       (2)

when PW is the pulse width of the defibrillating shock. The energy andresistance are then displayed on the front panel as shown in step 408and illustrated in FIG. 2 in blocks 88 and 90.

By supplying the physician with the measured resistance and energy,intelligent decision making can be carried out regarding subsequentenergy pulses. Clearly, if the resistance is much less than expected, itwould indicate a short in the leads. In addition, a physician can notethe actual energy delivered when preparing to deliver another shock atthis or a subsequent point in time.

As seen from FIG. 2, the front panel also includes a display block 92for illustrating the estimated energy. This display is intended tosupply the physician with information about the expected energy to bedelivered if the pulse that is presently selected on the LCD output ofthe panel were to be delivered. In order to make this calculation, thephysician must enter the expected resistance of the patient. Prior toany testing, this resistance will be set to a standard level. After someinitial testing, the physician may be able to provide a more accuratenumber for this resistance.

FIG. 9 illustrates the steps taken by the microprocessor to carry outthis function. As in the previous flow diagrams, the microprocessor willscan the front panel parameters in steps 500 and 502. If none of thedefibrillator panel switches have been changed, the other parametersswitches can be serviced in step 504. However, if any of thedefibrillator panel switches have been changed, a new estimated energymust be calculated. Note that any change entered by the physician to theexpected resistance, pulse width or voltage level will initiate thecalculation of a new estimated energy. The calculation of the estimatedenergy takes place in step 508.

The estimated energy can be calculated by using Equation (1) above whereVf (which was previously the residual voltage measured on the capacitor)is now approximated in the following equation:

    Vf=(Vi)exp[-PW/RC]                                         (3)

The variables listed above are the same as those discussed with relationto the display of actual resistance and energy. The resulting energy injoules is then displayed on the front panel as shown in step 510.

In summary, there has been provided a new and improved apparatus forassessing lethal ventricular tachyarrhythmia and in determiningdefibrillation thresholds. The subject invention advantageously combinesa programmable stimulator with a defibrillator. The apparatus isprovided with a number of features intended to improve operation,facilitating use by the physician and decreasing patient risk. Includedin these improvements is an automatic charging circuit coupled with adual channel capacitor defibrillator to substantially reduce the time todeliver a rescue shock if a test shock has failed. The subject systemfurther includes ability to store multiple parameters enabling the unitto be preprogrammed prior to initiation of cardiac procedure. The unitalso displays the energy and resistance present in a defibrillationshock. Finally, a display feature is also provided which givesinformation to the physician regarding the estimated energy of a pulseto be delivered having a specific voltage and pulse width.

While the above apparatus has been described with reference to apreferred embodiment, it should be apparent that various changes andmodifications could be made therein by one skilled in the art withoutvarying from the scope and spirit of the subject invention as defined bythe appended claims.

We claim:
 1. A defibrillation apparatus comprising:first storage meansfor storing and generating a first high voltage defibrillation pulse;second storage means for storing and generating a second high voltagedefibrillation pulse; input means for independently setting the voltagelevels stored in said first and second storage means; and means forselectively discharging the first and second storage means to providesuccessive, independent defibrillation pulses.
 2. An apparatus asrecited in claim 1 further comprising:means for regulating the energy insaid first and second storage means; and means for sensing the voltagelevels set on the input means and automatically enabling said regulatingmeans such that the energy in said first and second storage means is setto the respective sensed voltage level.